All Figures

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1652fig1.ps} \end{figure}$ Figure 1: Distribution of percentage of polarization p vs. intensity $I/I_{\rm max}$ at $\lambda =850$ $\mu$ m, normalized to its maximum value, for the starless cores L183 ( filled triangles, from Crutcher et al. 2004), L1544 ( empty triangles, from Ward-Thompson et al. 2000), and the B1-c core ( circles, from Matthews & Wilson 2002). The dashed lines are power-law fits, with slopes -1.6 for L1544, -1.2 for L183, and -0.4 for B1-c.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{1652fig2.ps} \end{figure}$ Figure 2: The geometry of the model. The H₀=1.25 singular isothermal toroid ( thick curves: isodensity contours; thin curves: magnetic field lines) is observed from a line of sight (l.o.s.) inclined by an angle $\theta$ with respect to the equatorial plane of the toroid ( $\theta =0$ : edge-on; $\theta =90^\circ$ : pole-on). Dust grains are assumed to be aligned with the local direction of the magnetic field, making an angle $\gamma$ with the plane of the sky (p.o.s.). Notice that for the inclination shown in this figure, magnetic field lines are almost in the plane of the sky for lines of sight intercepting the toroid above and below the centre, resulting in a relatively higher polarization degree of the outer parts of the core with respect to the central region.
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In the text

$\begin{figure} \par\includegraphics[angle=180,width=8.8cm,clip]{1652fig3.ps} \end{figure}$ Figure 3: Maps of the dust emission and polarization at $\lambda =850$ $\mu$ m for the H₀=0.2 singular isothermal toroid, shown in the bottom right panel ( solid curves, isodensity contours, dashed curves, magnetic field lines). The first three panels are for inclination with respect to the plane of the sky $\theta =20^\circ$ ( top left panel), $\theta =40^\circ$ ( top right panel), and $\theta =60^\circ$ ( bottom left panel). Each vector is proportional to the polarization degree (see scale on the lower right corner of each panel), and has been rotated by $90^\circ$ to show the average orientation of the magnetic field in the plane of the sky. The intensity is shown by contours logarithmically spaced by 0.2 dex starting from 10% of the peak value.
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In the text

$\begin{figure} \par\includegraphics[angle=180,width=8.8cm,clip]{1652fig4.ps} \end{figure}$ Figure 4: Same as Fig. 3 for the H₀=0.5 singular isothermal toroid.
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In the text

$\begin{figure} \par\includegraphics[angle=180,width=8.8cm,clip]{1652fig5.ps} \end{figure}$ Figure 5: Same as Figs. 3 and 4 for the H₀=1.25 singular isothermal toroid.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1652fig6.ps} \end{figure}$ Figure 6: Polarization degree as function of intensity at 850 $\mu$ m (normalized to the peak value $I_{\rm max}$ ) for the H₀=0.2, 0.5and 1.25 toroids for inclinations $\theta =20^\circ$ , $40^\circ$ and $60^\circ$ . The multiple tracks visible in each panel are an artifact due to the sampling of the polarization degree over a regular square grid in the polarization maps.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{1652fig7.ps} \end{figure}$ Figure 7: Variation of maximum ( solid line) and minimum ( dotted line) polarization degree as function of the isophotal aspect ratio r=b/a at $\lambda =850$ $\mu$ m for the H₀=1.25 toroid. The upper scale shows the inclination angle $\theta$ (in degrees) of the line of sight with respect to the equatorial plane of the toroid.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8.8cm,clip]{1652fig8.ps} \end{figure}$ Figure 8: Polarization degree as function of intensity at 850 $\mu$ m ( left panel) and 450 $\mu$ m ( right panel), normalized to the peak value $I_{\rm max}$ , for the H₀=1.25 toroid at $\theta =40^\circ$ . Notice the different extent of the "polarization hole'', here identified with the region with low and uniform values of p on the right side of the dashed lines: within $\sim$ 80% and $\sim$ 50% of the peak intensity at 850 and 450 $\mu$ m, respectively.)
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1652fig9.ps} \end{figure}$ Figure 9: Polarization degree as function of intensity at 850 $\mu$ m (normalized to the peak value $I_{\rm max}$ ) for the H₀=1.25 toroid at $\theta =40^\circ$ , in the case of external heating by the ISRF ( left panel) and assuming an isothermal dust temperature distribution ( right panel).
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In the text

$\begin{figure} \par\includegraphics[angle=180,width=8.8cm,clip]{1652fig4.ps} \end{figure}$	Figure 4: Same as Fig. 3 for the H₀=0.5 singular isothermal toroid.
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$\begin{figure} \par\includegraphics[angle=180,width=8.8cm,clip]{1652fig5.ps} \end{figure}$	Figure 5: Same as Figs. 3 and 4 for the H₀=1.25 singular isothermal toroid.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1652fig6.ps} \end{figure}$	Figure 6: Polarization degree as function of intensity at 850 $\mu$ m (normalized to the peak value $I_{\rm max}$ ) for the H₀=0.2, 0.5and 1.25 toroids for inclinations $\theta =20^\circ$ , $40^\circ$ and $60^\circ$ . The multiple tracks visible in each panel are an artifact due to the sampling of the polarization degree over a regular square grid in the polarization maps.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{1652fig7.ps} \end{figure}$	Figure 7: Variation of maximum ( solid line) and minimum ( dotted line) polarization degree as function of the isophotal aspect ratio r=b/a at $\lambda =850$ $\mu$ m for the H₀=1.25 toroid. The upper scale shows the inclination angle $\theta$ (in degrees) of the line of sight with respect to the equatorial plane of the toroid.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1652fig9.ps} \end{figure}$	Figure 9: Polarization degree as function of intensity at 850 $\mu$ m (normalized to the peak value $I_{\rm max}$ ) for the H₀=1.25 toroid at $\theta =40^\circ$ , in the case of external heating by the ISRF ( left panel) and assuming an isothermal dust temperature distribution ( right panel).
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